The present invention relates to a method of synthesizing bulk transition metal catalysts.
Carbides of transition metals such as Ti, Zr, V, Nb, Ta, Cr, Mo and W, along with their nitride and phosphide counterparts, have been recognized as catalytically active materials which could replace or substitute for existing process catalysts due to either better economics or performance. Potential applications include petroleum refining and biomass conversion. For example, research has shown that transition metal carbides, nitrides and phosphides can exhibit precious-metal-like catalytic behavior in a range of hydrocarbon conversion reactions such as hydrogenation, hydrogenolysis, and isomerization. Furthermore, under certain conditions, these materials compare favorably with CoMo or NiMo sulfides, which are a workhorse of the current petroleum refining as catalysts for hydrotreating processes which remove S, N, and O impurities from petroleum feedstock.
In most state-of-the-art catalysts used in petroleum refining and in the petrochemical industry, the active catalytic components or phases are finely dispersed on high surface area supports, for instance alumina and zeolites, to maximize the specific surface area (e.g., m2/g) of the active component. Supports are typically shaped as pellets or beads before deposition of catalytically active components. This shaping of powders into beads and pellets is necessary for practical catalytic processes which are generally implemented in large-scale fixed bed reactors. In fact, charging a large amount of catalyst powders in a reactor will lead to a densely packed bed, causing an excessive pressure drop and other technical issues.
In certain applications, deploying catalytic materials without supports (i.e., use as bulk catalysts) could be more advantageous. For example, having bulk catalysts could mitigate undesired reactions which involve support sites and/or interfaces between supports and active phases, thereby minimizing the formation of unwanted byproducts or deactivation species. The bulk catalysts could be particularly interesting in the emerging field of catalytic processing of biomass-derived liquids such as pyrolysis oils (also known as bio-oils), as conventional support materials and/or interfaces between supports and active phases are not structurally stable under the hot water-rich environments involved.
It has been recently shown that bulk transition metal carbides, nitrides and phosphides can be prepared with high surface areas via a temperature programmed reaction. In this procedure, by increasing the reaction temperature slowly and in a controlled manner in a flow of reducing gas mixture (e.g., CH4/H2 in the case of carburization), transition metal oxides are transformed to high surface area carbides, nitrides and phosphides. These materials are therefore an attractive candidate as bulk catalysts if they can be prepared in shapes adequate for practical applications (e.g., beads, pellets). Metal carbides are hard and refractory materials, which makes it difficult to shape carbides without losing attractive catalytic properties such as surface area (e.g., by high temperature sintering). There has been no known industrial application of shaped bulk carbide, nitride, or phosphide catalysts, but some methods have been proposed which involve activation of extruded metal oxide precursors (e.g., carburization or nitridation of Nb, Mo, W oxides). These prior art processes involve multiple steps to obtain bulk carbide and nitride pellets. First, acid forms of transition metals need to be prepared. The prepared metal acid powders are then mixed with celluloses, and the resulting mixture is peptized. The peptized product is extruded with an extruder. Following thermal treatments transform the transition metal acids to oxides, burn out cellulose, and increase mechanical strength of pellets. The temperature programmed carburization or nitridation of the pellets leads to high surface area bulk carbides and nitrides. So far the application of these prior-art processes has been limited to Mo, W, and Nb carbides and nitrides.